Coaxial transmission line termination employing tubular resistor cooled by internal and external annular water films



Oct. 19, 1965 H. E. HEDBERG 3,213,392 N LINE TERMINATION EMPLOYING TI ANNULAR WATER FILMS 2 Sheets-Sheet 1 Filed March 8, 1962 mm m0 mm mm m a E m w W Y H W no u R E I I. D Q m L T O A R M *w Y B N mm 5 6 MN 8 I llllil A l MK I m wv u m r m g m: L mm 2w mv I H Nu o Iv mO wk Ev I E I I M W I; .1 mm F 7 No. In m9 m mm mm g m9 4 Iv I I1 E. Nm 6. mm mm mm mm 5 mm m 1% mm I m h mm 5 mm l hx fll 1 1W- mm mm iv 8 5 Oct. 19, 1965 H. E. HEDBERG N LINE TERMINATION EMPLOYING TI 3,213,392 JBULAR COAXIAL TRANSMISSIO RESISTOR COOLED BY INTERNAL AND EXTERNAL ANNULAR WATER FILMS 2 Sheets-Sheet 2 Filed March 8, 1962 \MB n Ill l u HAROLD E. HEDBERG INVENTOR. BY Wm Q ATTORNEY United States Patent COAXIAL TRANSMISSION LINE TERMINATION EMPLOYING TUBULAR RESISTOR COOLED BY INTERNAL AND EXTERNAL ANNULAR WATER FILMS Harold E. Hedberg, Sunnyvale, Calif., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Delaware Filed Mar. 8, 1962, Ser. No. 178,360 8 Claims. (Cl. 33322) This invention relates to a coaxial transmission line termination and more particularly to a power absorbing load for the termination of high frequency coaxial transmission lines.

It is often desirable to dissipate high frequency energy in a load to prevent radiation of the energy. Also, it is often desirable to measure the amount of power dissipated in the load by the high frequency energy. The dissipation and measurement of high frequency energy is commonly effected by conversion of the radiant energy to heat energy, with the change of temperature produced by the heat providing an indication of the amount of power converted.

Coaxial loads for the dissipation of high frequency energy are well known and comprise, generally, inner and outer coaxial conductors. The inner conductor may comprise, for example, a tubular film resistor which includes a ceramic tube having a resistance film on the outer surface thereof, While the outer conductor may comprise a metallic tapered member surrounding the resistor and electrically connected thereto at one end. A solid dielectric material generally is disposed Within the space between the inner face of the tapered member and the resistance film. In the construction of contemporary wide band coaxial loads two liquid heat transfer media are often employed, i.e. an oil which is in direct contact with the resistive element for dissipation of the heat from the element, and water to which the heat from the oil is transferred. Such an arrangement, however, results in an undesirably long time lag between the generation of heat at the resistor and absorption thereof by the water.

In other prior art coaxial loads, cooling water is circulated through the hollow ceramic resistor for the dissipation of heat. However, the water is prevented from contacting the resistance film in such arrangements to prevent deleterious effects in the operation of the load. Again, a thermal time lag of undesirable magnitude results in the transfer of the heat from the resistive film, through the ceramic body of the resistor, to the water. In still another prior art construction, the solid dielectric material is spaced from the resistor to form an annular passage adjacent the resistance film for accommodation of a flow of dielectric fluid such as silicone oil through such passage to absorbe heat from the resistor. Water is not used as the dielectric fluid in such arrangements since large standing wave ratios in the line to the load device would result, making the device useless for its intended purpose. Prior art loads of the type including a dielectric fiuid in direct contact with the resistance film generally employ a fluid having a dielectric constant similar to the dielectric constant of the solid dielectric material. For example, it is common practice to use a solid dielectric body of material of the trade name Teflon together with a liquid dielectric material such as silicone oil, each of which has a dielectric constant of about 2.

Unlike such prior art coaxial load devices, the load device of this invention employs water, or a water solution, as the cooling medium, and the coolant is passed directly over the surface of the energy dissipation re- "ice sistor. The space between the resistor load device and the outer tapered conductor is occupied by a solid and a liquid having greatly different dielectric constants, the dielectric constant of the solid material being about 2 and the dielectric constant of the water, or water solution, being about 78. Numerous advantages result from the use of water as both a dielectric medium and the cooling medium.

An object of the invention is to provide a coaxial line termination for the dissipation of high frequency energy, having low thermal time lag.

Another object is to provide a coaxial line termination employing water as a coolant while providing good impedance matching between the transmission line and the termination.

These and other objects of the invention are achieved by providing a coaxial load in which water flows in direct contact with the resistance film of a cylindrical resistor, and in which the outer conductor of the coaxial load is tapered in such a manner as to avoid deleterious mismatching effects over wide frequency ranges.

FIGURE 1 is a simplified diagrammatic view of an arrangement which includes the termination in a radio frequency calorimeter;

FIGURE 2 is a longitudinal cross-sectional view of a termination embodying this invention;

FIGURE 3 is a perspective view of a rod-like insulating member which extends through the tubular resistor in the termination; and

FIGURE 4 is a simplified diagrammatic representation in longitudinal cross section, of a termination employing two dielectric materials between a cylindrical inner conductor and tapered outer conductor, which diagram is included for purposes of explaining the novel taper used in the outer conductor.

Reference is first made to FIGURE 1 of the drawings wherein there is shown in simplified form a diagram of a high frequency calorimeter comprising a water supply source 10 connected to the novel load device of this invention, designated 11, through a pipe or tube 12, having a valve 13 therein for control of the water flow through the device. Water from the source 10 circulates through the load and discharges therefrom through a tube 14. An important advantage of the load de vice 15 is that an open system for the fluid dielectric is economically feasible since the dielectric fluid may comprise water. Therefore, no cooling means such as a radiator and fan or refrigerating unit for the dielectric fluid is required.

A source of high frequency energy 16 is shown connected to the load device 11 through a transmission line 17. Power dissipated in the resistor of the load heats the resistor, which in turn heats the water flowing thereover. Thermocouples 21 and 22 are positioned at the water inlet and outlet, respectively, to the load device for providing potential outputs related to the temperature of the water entering and leaving the load device. By means of suitable apparatus, designated 23, the power flow into the load device may be directly indicated. Such apparatus may be of well known construction and include a supply potential source not shown, against which the outputs from the thermocouples 21 and 22 are balanced by adjustment of a potentiometer included in the balancing circuit and under control of a knob 24 at the panel of the apparatus. A meter 26 in the balancing circuit is adjusted for a null-balance condition and the position of the potentiometer knob provides an indication of the power dissipated in the load device. The valve 13 in the line 12 controls the rate of flow of the water through the load; the calibration of the potentiometer in the apparatus being dependent upon such flow rate.

Reference is now made to FIGURE 2 of the drawings wherein there is shown a longitudinal cross-sectional view of the load device 11, comprising a tubular film-type load resistor 31 which may comprise a cylindrical ceramic tube 32, with a resistive film 33 disposed on the external surface thereof. Said film may comprise an alloy including a highly stable tin oxide which is deposited on the external surface of the said glazed ceramic tube, which is then fired at a high temperature to disperse the resistance film throughout the body of the glaze.

The insulating tube 32 of the resistor is of a reduced outside diameter adjacent the ends thereof as at 34 and 34' and bands 36 and 36' of conductive metal such as silver, are applied to the ends and extend over the resistance film in good electrical contact therewith. It will here be understood that for purposes of illustration and clarity the showing of the resistance film 33 and conductive bands 36 are greatly exaggerated dimensionally in the drawing.

A generally cylindrical brass pin 39 formed with an integral axial flange 41 is conductively coupled to one end of the load resistor 31, the flange 41 extending over the silver band 36' and being secured thereto by soldering or other suitable means, not shown. The flange extends over the reduced diameter portion 34 of the resistor body and abuts the end thereof at an internal stepped portion, designated 42, to thereby limit the extent to which the resistor may be inserted into the flange. The outside diameter of the flange 41 is substantially equal to the outside diameter of the resistor 31 to permit a substantially unobstructed flow of water past the junction as will become apparent hereinbelow. The pin body 39 includes a series of progressively smaller diameter portions which terminate at one end thereof in a standard connector 43 for connection to the center conductor of a coaxial line such as the line 17 shown schematically in FIGURE 1.

The opposite end of the load resistor 31 is connected to the outer conductor, designated 46, of the load device through a hollow cylindrical connector 47. The tubular connector 47 fits over the rear end of the resistor and is suitably connected to the silver band 36 by soldering or other suitable means, not shown. A radial flange 48 is formed integrally on the connector 47 and fits within a recess 49 formed in the rear end wall of the outer conductor. A cylindrical rear end cover 51 of suitable insulating material is bolted to the end of the outer conductor by bolts 52 extending through clearance holes in the cover and engaging internally threaded holes in the outer conductor. A flexible O-ring 53 fitting within an annular recess 54 in the cover 51 abuts the rear end wall of the outer conductor to provide a fluid-tight seal between the conductor and cover.

The outer conductor body 46'is formed with an internal tapered wall 56 which surrounds and extends for the full effective length of. the resistance film of the resistor 31. The outer conductor body terminates at the front end thereof in an annular wall 57, to which is secured, as by bolts 58 (only one of which is shown), an annular heatinsulating spacer member 59. The bolts 58 seat within counterbored holes 61 formed in the spacer and extend rearwardly and outwardly therefrom for threaded engagement with tapped holes formed in the front end wall 57 of the outer conductor body. In addition, bolts 62 (only one of which is shown) seat in counterbored holes 63 and extend forwardly of the spacer. The bolts are non-rotatably secured to the spacer, as by means of knurled heads 64 which fit tightly the counterbored portions of the holes 63.

The bolts 62 extend into through-holes 66 formed in a front end connector body 67 and, together with nuts 68, secure the body 67 to the annular spacer member 59. A nipple 69 threadedly engages the connector body 67 for connection thereof to the outer conductor of the coaxial line, designated 17 in FIGURE 1. Threaded holes 71 (only one of which is shown), are formed in the body 67 for reception of mounting bolts for cooperation with a comple- '2 of the drawings.

l mentary member mounted on the outer conductor of the coaxial line and adapted for supplying high frequency energy to be dissipated.

The electrical path for the high frequency energy at the outer conductor of the coaxial line includes the nipple 69 and connector 67, each of which are made of electrical conducting material such as aluminum, brass, or the like. The heat insulating spacer 59 is made of material of poor thermal conductivity such as the electrical insulating material known as Bakelite. The ends of the annular spacer 59 and the inner diameter thereof are coated with metal 73 of high electrical conductivity, such as silver, which forms a low-resistance path for the high frequency energy between the connector body 67 and outer conductor body 46. Although the coating 59 is of good thermal conductivity, the thinness of the coating prevents excessive heat transfer therethrough. Thus, the body 72 with its low thermal conductivity, together with the thin coating 73 of silver thereon, comprise a heat insulating barrier for impeding heat transfer from the outer conductor body 46 to the connector body 67, while providing good electrical contact therebetween.

A solid dielectric body 76, made from an insulating material available under the tradenarne Teflon, fits within the tapered outer conductor body 46. The dielectric body 76 may be formed by molding, machining, or otherwise, and closely fits the conductor body 46. The

body 76 is formed with an internal bore 77 of slightly greater diameter than the diameter of the resistor 31, thereby forming a narrow annular passage, designated 78 to accommodate the flow of the fluid dielectric, water. The resistor is coaxially located within the bore 77 of the dielectric body, the coaxial relationship at the front end of the resistor being maintained by a close fitting relation between the bore and an external radial flange 81 formed on the inner end of the conductor 39.

The rear end of the resistor is fixedly positioned with respect to the outer conductor body 46 by the connector 47 secured to the resistor and held in fixed relation with the body 46 by the flange 48 which closely fits the recess 49 in the body. The tapered dielectric body 76, on the other hand, closely fits the tapered outer conductor, whereby the rear end of the resistor is coaxially located within the dielectric body. A flexible O-ring 82 fits between a radial wall of the flange 81 and the dielectric body 76 to provide a fluid-tight seal therebetween. In addition, an O-ring 83 fits in an annular groove 84 in the dielectric body 76 and abuts the outer conductor body 46 and the spacer 59 to provide a fluid-tight seal thereat.

A dielectric insert 86 is positioned in the space bounded by the stepped conductor rod 39, the stepped wall of the connector body 67 and the step formed by the said body 67 and the heat insulator 59. As is well understood by those skilled in this art, the insert 86 and pin 39 could be tapered instead of stepped in order to effect the desired change in diameter thereof between ends. The device is preferably constructed with a gap of approximately 0.01 inch between the dielectric body 76 and dielectric insert 86 to allow for axial expansion of the body 76 into said gap upon an increase of temperature of the body during operation of the device.

The tube 12 (FIGURE 1) for the supply of water to the load device is adapted for connection to a tube insert 91 comprising a portion of a male fitting 93 threadedly engaging the outer conductor body 46. An annular recess 94 is formed in the bore of the body 46 adjacent the rear end thereof, which recess communicates with the fitting 93 through a passage 96 between the said annular recess 94 and the recess 97 within which the connector 93 is threaded. The direction of flow of the water dielectric is indicated by arrows designated 99, in FIGURES 1 and As seen in FIGURE 2, water from the fitting 93 flows through the passage 96 in the body 46 and into the annular recess 94. From the recess 94 the water flows from the rear of the device to the front end thereof along the surface of the resistor film 33 in the narrow annular passage 78. Radial holes 101 are formed in the flange 41 of the pin body 39, through which holes water from the annular passage 78 flows to the interior of the tubular resistor 31. A narrow tubular passage, 102, along the inner wall of the tube 32 for the return flow of the water therethrough is formed by the coaxial positioning of a rod 103 within the said resistor tube 32. Rod 103 is formed of a low dielectric material having a low thermal conductivity, such as the material known by the tradenarne Rexolite, or the like. The rod has a diameter slightly less than the inside diameter of the resistor to thereby form the said annular passage 102. The front end of the rod is provided with a threaded reduced diameter portion 104 which cooperates with a tapped coaxial hole 106 formed in the inner end of the pin body 39. The rod is shown in perspective view in FIGURE 3 of drawings to which figure reference is also made. The rear end of the rod is provided with a radial flange 107 which engages the inner wall of the tubular resistor body to maintain coaxial relation between the resistor and rod. Axial flutes, or grooves, 108 are formed in the end of the rod 103 through the flange 107 to provide passages communicating between the annular passage 102 and the rear end of the resistor. An axial bore 109 is formed in the rear end cover 51, and a male fitting 93' cooperates with an enlarged diameter threaded portion of the bore, through which the water dielectric discharges from the device.

In the device illustrated in FIGURE 2 a plastic sleeve 111 surrounds the rear end cover 51 and the major portion of the outer conductor body 46, with a bolt 112 securing the sleeve to the body. Such sleeve serves to thermally insulate the device. Also, a generally V-shaped annular groove 113 is formed in the outer wall of the outer conductor body for weight reduction.

It has been determined that, by suitable tapering of the outer conductor wall 56, good matching of the load device to the transmission line can be obtained so as to provide a voltage standing wave ratio of about unity. Such results are obtained using a taper substantially as specified by the following equation:

where x is the distance along the resistor 31 from the rear end thereof, Y

D is the total effective length of the resistor 31,

R is the total resistive value of the resistor 31,

0 is the radius of the tapered solid dielectric body 76 at a distance x along the resistor,

a is the radius of the resistor 31,

b is the radius of the axial bore 77 through the tapered solid dielectric body 76,

6 is the dielectric constant of the dielectric liquid (water) flowing through the passage 78, and

e is the dielectric constant of the tapered solid dielectric body 76.

Reference is made to FIGURE 4 of the drawings wherein there is diagrammatically shown, in simplified form, a portion of the novel two-dielectric termination, for the purpose of identifying the various parameters in the equation. By the above equation the proper radius c at all points x distance from the rear end of the load resistor may be determined, and a termination having dimensions which satisfy the equation will provide a substantially unity voltage standing wave ratio.

One of the requirements of construction is that the thickness of the water film outside the resistor be made thin. In practice accurately dimensioned passages 78 and 108 of about .02 inch have been found suitable. If the water passage 78 is too thick, spurious radio frequency modes would be excited. (The device normally operates in the TEM mode, as in the usual manner.) The water passage 102 is also kept thin to minimize reflection of energy from the device. Despite the thinness of the water blanket, adequate cooling of the load is effected since the thermal conductivity of water is relatively great being about four times that of silicone oil which is commonly used in fluid dielectric cooled loads.

My invention now having been described in detail in accordance with the requirements of the patent statutes, various changes and modifications will suggest themselves to those skilled in this art, and it is intended that such changes and modifications shall fall within the spirit and scope of the invention as recited in the following claims.

I claim:

1. A microwave energy dissipating apparatus comprising, a tubular film-type resistor which includes a resistance film at the outer wall of a tube of insulating material, a hollow tapered outer conductor body surrounding said resistor and electrically connected thereto at one end, a tapered solid dielectric body having an axial bore therethrough positioned within the tapered outer conductor between the tapered outer conductor and resistor, the wall of said bore being spaced from the said resistance film to form a thin annular passage, said tapered solid dielectric body substantially completely filling the space between said tapered outer conductor and said tubular resistor except for said thin annular passage, a generally cylindrically shaped rod of dielectric material coaxially positioned within said tubular resistor and spaced therefromto form a thin annular passage, means forming a passage interconnecting said thin annular passages ad jacent one end of said resistor, and first and second means forming passages for introduction and discharge, respectively, of a stream of dielectric liquid to and from said thin annular passages.

2. A microwave energy dissipating apparatus comprising a hollow tapered outer conductor, a cylindrical resistor therewithin, a tapered solid dielectric body positioned within the tapered outer conductor between the tapered outer conductor and resistor and having an axial bore therethr-ough to receive said resistor, the wall of said bore being spaced from said resistor to form a thin annular passage, and means forming a passage for the introduction of a stream of dielectric liquid into said thin annular passage, the dimensions of the apparatus satisfying the following equation:

60D ln where x is the distance along the resistor from one end thereof,

D is the total length of the resistor,

R is the total resistance of the resistor,

c is the radius of the tapered solid dielectric body at a distance x along the resistor,

a is the radius of the resistor,

b is the radius of the axial bore through the tapered solid dielectric body,

6 is the dielectric constant of the dielectric liquid, and

Ed is the dielectric constant of the tapered solid dielectric body.

3. The invention as recited in claim 2 including an annular spacer of insulating material having a coating of electrically conducting material thereon, a connector body, and means securing the connector body to the outer conductor body through said spacer, said spacer providing a thermal barrier between said outer conductor body and said connector body while electrically connecting the same.

4. Apparatus as recited in claim 2 wherein said tapered solid dielectric body substantially completely fills the space between said tapered outer conductor and said cylindrical resistor except for said thin annular passage.

5. Apparatus as recited in claim 4 wherein the smaller end of said tapered outer conductor is adjacent and electrically connected to one end of said cylindrical resistor.

6. In a microwave energy dissipating apparatus which includes a hollow tapered outer conductor and a cylindrical resistor therewithin, a tapered solid dielectric body having an axial bore therethrough positioned Within the tapered outer conductor between the tapered outer conductor and resistor, the wall of said bore being spaced from the said resistor to form a thin annular passage, and means introducing a stream of water into the said thin annular passage, the dimensions of the apparatus satisfying the following equation:

[6 In g -Fed ln 2] where ing, a tubular film-type resistor which includes a resistance film at the outer wall of a tube of insulating material, a hollow tapered outer conductor body surrounding said resistor and electrically connected thereto at one end, a tapered solid dielectric body having an axial bore therethrough positioned Within the tapered outer conductor between the tapered outer conductor and resistor, the wall of said bore being spaced from the said resistance film to form a thin annular passage, 21 generally cylindrically shaped rod of dielectric material coaxially positioned within said tubular resistor and spaced therefrom to form a thin annular passage, means forming a passage interconnecting said thin annular passages adjacent one end of said resistor, and first and second means forming passages for introduction and discharge, respectively, of a stream of dielecrtic liquid to and from 3 said thin annular passages, the dimensions of the apparatus satisfying the following equation:

in ln -i-ed ln a b (1 where:

x is the distance along the resistor from one end thereof,

D is the total length of the resistor,

R is the total resistive value of the resistor,

60;) is the radius of the tapered solid dielectric body at a distance x along the resistor,

a is the radius of the resistor,

b is the radius of the axial bore through the tapered solid dielectric body,

6 is the dielectric constant of the dielectric liquid, and

e is the dielectric constant of the tapered solid dielectric body.

8. A microwave energy dissipating apparatus comprising, a hollow tubular resistor, a hollow tapered outer conductor body surrounding said resistor, a tapered solid dielectric body positioned within the tapered outer conductor between the tapered outer conductor and resistor and having an axial bore therethrough to receive said resistor, the wall of said bore being spaced from the outer surface of said resistor to form a first thin annular passage, said tapered solid dielectric body substantially completely filling the space between said tapered outer conductor and said tubular resistor except for said thin annular passage, a generally cylindrically shaped rod of dielectric material coaxially positioned within said tubular resistor and spaced therefrom to form a second thin annular passage, means forming a passage interconnecting said first and second thin annular passages adjacent one end of said resistor, and first and second means forming passages for introduction and discharge, respectively, of a stream of dielectric liquid to and from said thin annular passages.

References Cited by the Examiner UNITED STATES PATENTS 2,274,381 2/42 Richardson 338- 2,412,462 12/46 Marsten 338-55 2,677,743 5/54 Canegallo 33855 2,894,219 7/59 Frederico 333-22 2,966,639 12/60 Heller 33322 3,044,027 7/62 Chin 33322 HERMAN KARL SAALBACH, Primary Examiner.

Patent No, 3,215,392 October 19, 1965 Harold E. Hedberg It is hereby certified that e ent requiring correction and tha corrected below.

rror appears in the above numbered patt the said Letters Patent should read as Column 5, lines 43 to 46,

for that portion of the formula reading 80D 60D R rea R column 6, lines 48 to 51, column 7,

lines 16 to 19, column 8, lines 3 to 6, for that portion of the formula, each occurrence, reading column 5, lines 43 to 46, column 6, lines 16 to 19, and column 8, of the formula,

lines 48 to 51, column 7,

lines 3 to 6, for that portion each occurrence, reading (X) read c Signed and sealed this 26th day of July 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A MICROWAVE ENERGY DISSIPATING APPARATUS COMPRISING, A TUBULAR FILM-TYPE RESISTOR WHICH INCLUDES A RESISTANCE FILM AT THE OUTER WALL OF A TUBE OF INSULATING MATERIAL, A HOLLOW TAPERED OUTER CONDUCTOR BODY SURROUNDING SAID RESISTOR AND ELECTRICALLY CONNECTED THERETO AT ONE END, A TAPERED SOLID DIELECTRIC BODY HAVING AN AXIAL BORE THERETHROUGH POSITIONED WITHIN THE TAPERED OUTER CONDUCTOR BETWEEN THE TAPERED OUTER CONDUCTOR AND RESISTOR, THE WALL OF SAID BORE BEING SPACED FROM THE SAID RESISTANCE FILM TO FORM A THIN ANNULAR PASSAGE, SAID TAPERED SOLID DIELECTRIC BODY SUBSTANTIALLY COMPLETELY FILLING THE SPACE BETWEEN SAID TAPERED OUTER CONDUCTOR AND SAID TUBULAR RESISTOR EXCEPT FOR SAID THIN ANNULAR PASSAGE, A GENERALLY CYLINDRICALLY SHAPED ROD OF DIELECTRIC MATERIAL COAXIALLY POSITIONED WITHIN SAID TUBULAR RESISTOR AND SPACED THEREFROM TO FORM A THIN ANNULAR PASSAGE, MEANS FORMING A PASSAGE INTERCONNECTING SAID THIN ANNULAR PASSAGES ADJACENT ONE END OF SAID RESISTOR, AND FIRST AND SECOND MEANS FORMING PASSAGES FOR INTRODUCTION AND DISCHARGE, RESPECTIVELY, OF A STREAM OF DIELECTRIC LIQUID TO AND FROM SAID THIN ANNULAR PASSAGES. 